1. Field of the Invention
The present invention relates to an optical pickup equipped with an aberration correction mechanism for correcting spherical aberration.
2. Description of Related Art
An optical disc apparatus performs recording or reproduction of information by projecting a laser beam from an optical pickup to an optical disc as a recording medium. A groove called a track is formed on a recording surface of the optical disc. The optical pickup condenses the laser beam to be a laser spot on the track or receives the laser beam reflected by the track so that the information is recorded or reproduced.
A width of the track on a compact disc (CD) medium is different from that on a digital versatile disc (DVD) medium having larger record information quantity than the CD medium, so a diameter of the laser spot when information is recorded or reproduced is also different between them. The laser spot has a circular shape having a diameter that is proportional to a wavelength of the laser beam and is inversely proportional to a numerical aperture (NA) of the objective lens.
Recently a BD (Blu-ray Disc) medium came on the market, which has record information quantity larger than that of the DVD medium. The BD medium has a track width smaller than that of the DVD medium, and a diameter of the laser spot for the BD medium should also be smaller than that for the DVD medium. Therefore, a laser beam of blue violet color having a wavelength (of approximately 405 nm) shorter than that of the laser beam for DVD is used for recording or reproducing information on a BD medium. In addition, although the NA of the objective lens for DVD is approximately 0.6, an objective lens having higher NA that is approximately 0.85 is used.
The spherical aberration of the objective lens increases in a proportional manner to a biquadrate of the NA of the objective lens and a thickness of a transparent layer of the optical disc. Therefore, there is a problem that if the NA of the objective lens is increased so as to decrease a diameter of the laser spot, the spherical aberration will become larger than the case of using an objective lens having a smaller NA. For instance, comparing the objective lens for DVD (NA=0.6) with the objective lens for BD (NA=0.85), quantity of spherical aberration becomes four times larger in case of using the objective lens for BD than in case of using the objective lens for DVD.
If the spherical aberration increases, the laser spot will be blurred, and intensity of light at the midmost decreases. If the spot becomes blurred, it is difficult to project the laser beam onto the track correctly and to read information recorded on the recording surface. In addition, if the intensity of light at the midmost decreases, it becomes difficult to record information. Since the laser beam emitted from the light source is increased for securing the recording, it becomes difficult to record on a small area (track). In addition, a cover layer of the optical disc has a standardized thickness, but an actual thickness of the cover layer through which the laser beam passes is not uniform due to a manufacturing error or a wobbling of the disk. A variation of the thickness of the cover layer also causes a variation of the quantity of spherical aberration.
Therefore, a spherical aberration correction mechanism is used, in which an aberration correction lens is moved before the objective lens so that the spherical aberration can be corrected. This spherical aberration correction mechanism for correcting the spherical aberration includes a movable portion for supporting a plurality of aberration correction lenses in a movable manner in the optical axis direction of the laser beam, a feed screw having a helical groove on the outer surface disposed in parallel with the optical axis of the laser beam and an engaging portion for engaging with the feed screw. The engaging portion is made to engage with the feed screw. In this state the feed screw is driven by an electric motor to rotate so that the engaging portion moves linearly in the direction along the center axis of the feed screw. Thus, the aberration correction lenses supported by the movable portion can be driven to slide in the optical axis direction of the laser beam. When the aberration correction lenses are moved to slide so that a distance between the aberration correction lenses is adjusted, the spherical aberration is corrected. Thus, the spherical aberration generated when the laser beam passes through the transparent layer of the optical disc can be reduced when it passes through the objective lens.
The conventional optical pickup uses a drive mechanism of the movable portion, which includes the engaging portion with teeth having protrusions so that the teeth can engage with the helical groove of the feed screw. In this drive mechanism, the engaging portion is pressed by a leaf spring to the feed screw in the radial direction of the feed screw so that the teeth and the feed screw are engaged with each other without a gap. In this way, the teeth are pressed to the feed screw continuously so as to suppress an occurrence of a so-called backlash that is a timing difference between rotation of the feed screw and an action of the teeth when the teeth are pressed by the rotation of the feed screw. Thus, when the feed screw rotates, a knife edge also moves to slide so that the movable portion can be moved to a correct position in a short time (see JP-A-2003-45068, JP-A-2007-18680 and the like).
In addition, there is another optical pickup recently, in which a lead nut engaging with the feed screw is used for the lens driving portion of the spherical aberration correction mechanism instead of the teeth. In this case where the lead nut engages with the feed screw, there are merits compared with the one using the teeth, which include stable accuracy of positioning in the normal operation, little dropout of engagement, high productivity of the teeth shape of the component and the like.
However, in the case of the one using the lead nut as the drive mechanism of the movable portion, electric power may be supplied to the drive motor continuously after the lead nut reaches the end portion of the feed screw. If the feed screw is driven to rotate continuously, the screw thread of the lead nut and the screw thread of the feed screw may be seized up with each other so that they cannot be reset to the initial state even if the feed screw is moved in the opposite direction by the drive motor.
Furthermore, some of the conventional optical pickups include a cushioning member made of urethane foam or the like disposed at a motion limit of the movable portion. However, it can relieve an impact due to a collision but cannot prevent occurrence of the seizing up and the malfunction of being unable to reset after the seizing up.